Surface acoustic wave (SAW) filters are used predominantly in GSM mobile telephones. SAW filters are operated single-ended on an input side and balanced on an output side. Single-ended means that a signal is applied to only one of two connectors, while the other connector is grounded. This method of connection is also referred to as asymmetric. An input or output of a SAW filter that is operated in a balanced mode, on the other hand, has two connectors whose signals are ideally phase-shifted by 180° relative to one another. This means that a signal that is equal in magnitude can be obtained at both connectors, and that the signal differs merely in its prefix. Such symmetric/asymmetric operation is also referred to as a BALUN function.
For more modern mobile radio systems, such as EDGE, UMTS and CDMA, the market is increasingly demanding SAW filters that can be operated in balanced mode on both sides. Such filters are already being installed in numerous established mobile radio systems under AMPS, PCS, and PDC 1.5.
Another critical factor for the mode of operation of a SAW filter is the filter's impedance. Until recently, a filter impedance of 50 ohm on the input side and the output side was consistently being demanded and offered, while now, higher impedance values in the range of 100 to 400 ohm are increasingly required for balanced-balanced filters.
A SAW HF (high frequency) filter must also satisfy stringent requirements with regard to selection and insertion attenuation, particularly in the realm of mobile communication. These requirements can only be met with special, novel filter structures.
Known filters that can be operated in balanced mode on both sides are known, for example, from EP-A-0 605 884. These filters are implemented using longitudinal mode resonator filters (=dual mode SAW=DMS filters) on lithium niobate or lithium tantalate. In these filters, an odd number of interdigital transducers is disposed between two reflectors for each track. A balanced-balanced filter is structured, for example, as a two-track filter, in which two tracks with three transducers are respectively connected in cascade via the center transducer. The two outer transducers of a track are respectively connected to the input or output, and demonstrate a phase inversion, which permits symmetric operation at each input or output. Such a filter possesses the same impedance on the input and the output sides.